Thermal energy generators are expected to be phased out and not replaced as they age out. However, the biggest issue to be resolved will be the erratic generation from renewable sources, which is not controllable on demand. Solar plants operate only when sun is shining, while wind mills operate only in windy conditions. Renewable energy will have to necessarily depend on devices and facilities which can store the energy when available and dispense to loads as per demand...

- R. P. Deshpande

Energy storage is thus a basic requirement for deep penetration of renewable energy. Even at today’s status, problems of energy management (both generation pattern and peak load) need serious attention. With installed capacity of 14%, total contribution from renewable energy generation in India is limited to just 5.6%. India’s renewable energy target of 175 GW by 2022 target is not uniform state wise, but depends on local factors. This involves transmission of power over long distances. Issues needing consideration relate to storage, dispatch and distribution, peak loads and transient loads in order to target total dependence on renewable energy. With a mix of thermal, hydro, nuclear and renewable sources, load distribution and dispatching are not easy to manage.

Downward costs of PV and wind energy, and rising electricity prices are also contributing to spread of renewable energy. Governments all over the world are encouraging these developments by way of incentives. Solar and wind power mix on an extended electricity grid has to match total demand and intermediate electricity storage throughout the year.

Today applications for energy storage include load following, renewable energy grid integration, and renewable energy time shifting. In future, time of use energy cost for commercial and industrial segments and conventional energy time shifting will also be part of management. Pike Research forecasts worldwide revenue growth for stationary energy storage systems for the electricity grid a strong pace, increasing from $1.5 billion in 2010 to $35.3 billion annually by 2020.

Traditional options for long duration energy storage include pumped hydroelectric storage, compressed air energy storage (CAES), and sodium sulfur (NAS) batteries. Other energy storage technologies are lithium ion batteries (energy density of 100-400 KWH/Kg), flow batteries, hydrogen generation and storage, and hydrogen based batteries like Ni-MH and Ni-Fe batteries with very long lifetimes. Ammonia based energy storage having energy density of 6.5 KW/Kg is also getting attention. Pike Research forecasts the most significant growth opportunities for CAES, Li-ion batteries, and flow batteries, although there are other prevalent technologies as well.

Batteries convert electricity into chemical energy for storage and back into electrical energy, and perform different functions at various points along the electric grid. At solar PV or wind turbine sites, batteries can smooth out the variations in supply, store excess energy when demand is lean and release it when demand is high. Batteries can store energy when it exceeds a local substation's capacity and release the power when the flow is less; or store energy when prices are low, so it can be sold back to the grid when prices rise. For households, batteries can store energy for use anytime and provide back-up power in case of blackouts.

Cost of battery energy storage (a major concern) is going down. Growing EV markets, benefitting from incentives and large scale manufacturing, are helping to bring down battery costs. For a reliable and steady supply and demand balancing, utilities today handle fluctuations in solar and wind power by adjusting their operations. As the amount of renewable energy grows, better battery storage is crucial. Batteries performance and safety issues limit their full integration in grid systems, along with regulatory barriers and costs, while researchers around the world are working towards better and cheaper batteries.

World over today major dependence is on battery storage- both lithium ion and lead acid types. However, presently there are practical limitations to their storage capacity. There have been persistent efforts to increase storage of battery systems, and the largest system installed today is 40MW.

World’s largest battery storage system record is held by the State Grid Corporation of China with a 36MWh battery (less than 1/10 the size). This will be surpassed by 400MWH Li- ion battery-based energy storage facility near completion in South California capable of providing 100MW of power for up to four hours. Even this system is not enough for a large size distribution grid, but is sufficient to buffer conventional supplies at peak load and reduce operating costs.

International Renewable Energy Agency projects that by 2030, solar PV capacity will be grown nine times its size in 2013, while wind power could go five-fold. On the vehicle front, BEVs have range generally not exceeding 150 km, though one model of Tesla has a range of 240 miles (400 km). EV sales have increased 128% since 2012 and plug-in market is expected to grow at about 16% p.a. till 2024.

Tesla has embarked on one of the largest Lithium battery manufacturing units in order to bring down costs for their vehicles, as also to meet grid storage demand. Conventionally, battery storage cost lies anywhere between $300-$500/KWH. Tesla is aiming at $100 per KWH at this Gigafactory in near future from its current price of $150-$200 / KWH with some design change and also due to bulk manufacture. Another company Bio Solar is working on a breakthrough technology to bring it down substantially to a level target level of $54/KWH. Battery costs are fast heading downwards- a healthy sign for energy storage.

Li-ion batteries start degrading typically after around 1,000 charge / discharge cycles. So, long term cost of ownership per KWH produced may be found by spreading the cost of battery over 1,000 cycles. Considering the efficiency of the battery, (Li-ion is around 90% efficient), this may be increased by 11% to get effective cost. Long term Levelized Cost of Energy (LCOE) of battery can be estimated as follows:

If carbon-free energy is to be cheaper than coal or natural gas, batteries should be extremely cheap in terms of cost per KWH produced. Battery system costs are naturally the final purchase costs, including installation and maintenance. Wholesale grid electricity in the US at base load hours in night averages 6-7 cents / KWH, while retail rates average around 12 cents per KWH.

Battery storage can probably solve some issues faced by the penetration of solar and wind technology in the short term. Other technologies are emerging as potential long-term solutions, and proponents of these contend that they will be cost-competitive.

B] Compressed Air Energy Storage

Compressed air energy storage, also known as CAES, is ideal for utility from 10 MW to 100 MW. It requires underground storage in natural or man-made caverns, and can work for storing wind or solar energy outputs. Adiabatic compressed air energy storage (CAES) uses tanks and compressors that are capable of 30 years or more of continuous use (over 10,000 cycles) with complete discharge capability (compared with 70% discharge for in Li-ion). Further, CAES can be used to store energy from a few weeks to years.

General Compression, New Zealand, is working on a model that will provide around 20-40 MWH of storage for each MW of peak power production. For a 100 MW wind project, the ideal would be a facility between 200 MWH and 400 MWH of storage. The company expects CAES would be able to deliver this at one fourth the price of battery technologies.

C] Pumped Hydro

Pumped hydro conventionally supports coal and nuclear energy to meet short-term changes in demand. The system can also be used to absorb and manage changes in supply from renewable sources. Benefits include stabilizing and reducing wholesale electricity prices, increasing the spread of renewable energy and improving grid operations. A new approach locates pumped hydro storage away from natural watercourses using natural contours to situate two reservoirs at different elevations that could be used to store energy, thus obviating the need to curtail output from wind farms.

Pumped hydro storage is efficient, flexible, economical and commercially available on a vast scale and is the only large-scale storage technology currently available to the utility industry. Competing storage techniques such as compressed air, high-temperature thermal storage in conjunction with concentrating solar thermal and advanced batteries are considerably costlier or less developed. There are only around 200 large pumped hydro systems in the world with a total capacity of around 130 GW.

D] Flow Batteries

Flow batteries, a newcomer in the market, can theoretically operate for over 5,000 to 10,000 cycles or more. This technology, akin to both a fuel cell and a battery, uses liquid electrolytes to create electricity. The liquid that can be replaced fast and economically, and the spent liquid so removed can be recovered outside. It may also be recharged electrically. Fundamental difference between conventional batteries and flow cells is that energy is stored at the electrode material in conventional batteries but in the electrolyte in flow cells.

In redox flow batteries (RFB),“redox” refers to chemical reduction and oxidation reactions to store energy in liquid electrolyte solutions circulating in electrochemical cells during charge and discharge. Separation of power and energy is a key feature of RFB’s. Energy is stored in the volume of electrolyte, which can range from a few KWH to tens of KWH, depending on the size of the storage tanks. Power capability is determined by the size of the stack of electrochemical cells in reaction chamber. Energy rating corresponds to discharge at rated power for two to eight hours. Flow can easily be stopped during fault conditions. This limits system vulnerability to uncontrolled energy release to a few percent of the total energy stored. This scores over integrated cell storage batteries (lead-acid, NAS, Li Ion), where full stored energy is always available for discharge in case of fault. The storage tanks can be located away from reaction chamber, and tanks can be huge to match required total energy storage. This permits considerable flexibility in design depending upon available space and geometry.

Redox flow electrolytes being very cheap (0.1$/kg for aqueous types) can be a serious competitor of gasoline. Further, flow batteries are known for longevity, with companies already offering 20 years warranty. Nanoflowcell has been exhibiting a sports car using their flow batteries. Pure electric cars using the 48V batteries are said to have a range of 1,000 miles.

New York is planning a storage capacity of 100 MWH by 2020 along with an expanded solar target of 1,000 megawatts by 2030. Energy storage, with its capacity to integrate variable wind and solar power sources into the grid, is expected to play a critical role in meeting the city's plans by 2050. The planning includes a 400 KWH Vanadium redox flow battery system in Manhattan.

Today, the most advanced flow batteries are vanadium redox batteries (VRBs), which store charges in electrolytes that contain vanadium ions dissolved in a water-based solution. Vanadium’s advantage is that its ions are stable and can be cycled through the battery over and over without undergoing unwanted side reactions. Since VRBs have a relatively low energy density, the external tanks have to be quite large to hold enough power. Other flow battery types include Iron- Chromium flow batteries and Zinc-Bromine flow batteries.

E] Molten Salt Energy Storage

Molten salt is among the most flexible, efficient and cost-effective form of large scale energy storage system deployed today. Molten salt energy storage, a mixture of 60% sodium nitrate and 40% potassium nitrate (which is the most commonly used salt), stores energy at over 55 °C in a hot molten salt storage tank until electricity is needed – day or night, during off-peak/normal hours or peak hours. When electricity is needed, the hot salt is pumped to a conventional steam generator to produce superheated steam for a turbine/generator as used in any conventional coal, oil or nuclear power plant. Typically, a 100 MW turbine would need a tank of about 9.1 meters tall and 24 meters in diameter to drive it for four hours.

Storage of energy in molten salt increases the reliability of solar systems, allows the plants to operate 24x7 providing base load power for both on-grid and off-grid applications, increasing the net annual output from a solar energy plant. It helps to avoid expensive fuel costs and provides cleaner power generation. Global molten salt energy storage capacity was about 1,290 MW in 2014 and is expected to reach 3,841.0 MW by 2018.

Molten salt storage enables solar thermal power plants to generate electricity when it is needed, as in conventional systems. Plants with such energy storage can operate 24x7 for both on-grid and off-grid applications. Molten salt thermal energy storage is the lowest capital cost energy storage system. It can prove as one of the most cost-effective solution for CSP thermal power generation.

Figure 5: Flywheel construction (Source: Beacon Power, LLC)

F] Hydrogen Storage

Hydrogen is the most versatile material for storage. It can be produced and stored in both small and large scale. Small amounts of hydrogen (up to few MWH) can be stored in pressurized vessels at 100-300 bars or in liquefied state at low temperatures. Large quantities can be stored in underground caverns up to 50,000 cubic meters at 200 bars enough to produce 100 GWH of electrical energy.

As per Fraunhofer Institute for Solar Energy Systems in Freiburg, Germany, hydrogen and methane are the only options for large scale storage. According to them, pumped hydro and battery can at best provide storage capacity up to 50 GWH, but a tremendous amount of long-term storage up to 70 terawatt-hours can only be done with hydrogen and methane. They plan to use caverns to store hydrogen, which can be used for vehicles or in fuel cells. Alternatively, it can be converted into methane.

Electrical energy can be converted to hydrogen by electrolysis. Stored hydrogen can be converted back to electricity by using various alternatives. Round trip efficiency today is as low as 30- 40%, and may increase to 50% if more efficient technologies are developed. Still, interest in hydrogen storage is growing due to much higher storage capacity at 120 MJ/Kg compared with 44 MJ/Kg for gasoline.

Reconversion to electrical energy can be done via fuel cells with efficiency of 50% or by burning in power plants at still higher efficiency of 60%. Hydrogen can additionally be used as feedstock for chemical or petroleum industry or in fuel cell vehicles.

While hydrogen storage is amenable to large scale grid systems, its use in fuel cell vehicles is a challenge to extend the range to over 450 km (comparable to gasoline vehicles). Though fuel cell vehicles of large size or luxury market have been developed, the real issue is its development for small car segment. This stems from the fact that due its low density, hydrogen storage space requirement is 8 MJ/l as against 32 MJ/l for gasoline.

Latest news from Delft University of Technology, Netherlands has come up with a new device called 'battolyser'. This integrated battery electrolysis system can not only store or supply electricity efficiently as a battery, but can also electrolyze water into hydrogen and oxygen. The device is based on Ni-iron battery technology, whereby storage as well as hydrogen production are both efficient. It can store and supply electricity efficiently as a battery, and when the battery is fully charged, it automatically starts splitting water into hydrogen and oxygen. Combining the two technologies, efficiency as high as 90% is expected to be achieved, and it could be a new milestone in energy storage costs. The researchers hope to scale up their model for grid level storage in near future.

Short time storage requirements:

The technologies above are gearing up for large scale energy storage needed for grid level and automobiles. In practice, transient loads at grid level cannot be met fast enough by batteries, and in automobiles there are problems leading to deep discharging of batteries. In both cases, battery life is adversely affected. In industry and grid supplies, if the transients are not met in the small time limits, it can lead to frequency and voltage fluctuations, and even the entire grid may collapse in extreme case. This battery limitation comes in because of the process of converting chemical energy into electricity and consequent time lag. Other modes of storage are also not capable to adjust within a few seconds or minutes.

Two technologies developed in recent years, viz ultracapacitors and high speed flywheel storage can take care of such transient and short time requirements. Both have very fast response time and can efficiently fill up this short time need ranging from a few milliseconds to a few minutes. While ultracapacitors have been more common, flywheel storage is making entry in grid or industrial applications.

Use of these systems can take care of small time requirements in case of power breakdown, when enough time is made available for an orderly shutdown of load, or to switch over to an alternative supply source like a DG set. They can smoothen out supply variations in solar or wind energy sources on account of inevitable natural factors like passing clouds, change in wind speeds etc. At the same time, they help absorb excess power fast and deliver it back equally fast whenever required.

G] Ultracapacitors(also called Supercapacitors)

Advent of ultracapacitors in recent years has enabled very high energy storage, comparable to battery storage (though most of these today store much less energy than battery). Energy is stored in ultracapacitors as electrical energy and is given off as such, without any time gap or energy conversion. As a result, the power an ultracapacitor system can discharge is several times that of battery, and the millisecond response takes care of all transients seen by grid systems or automobiles. Further, the round trip charge / discharge efficiency of the system is over 95-98% (lead acid battery efficiency is 65-70%).

Ultracapacitors can withstand tens of thousands or millions of charge/discharge cycles without losing their energy storage capacity. They outlast the lifetime of equipment or vehicles in which they are used. Ultracapacitors bridge the energy gap between traditional capacitors with high power output, and batteries with high energy storage capacity. When paired with battery, they extend battery life several folds by taking over all jerky loads.

Ultracapacitors perform various functions in automobiles like vehicle starting, jump start and cold start, power steering, window operation, regenerative braking and so on. Railways are benefiting by using them for recovery of kinetic energy from decelerating train and reusing the same for accelerating units. Huge energy saving up to 25% or more is being achieved by metros in Korea and elsewhere using ultracapacitor based trackside storage systems. Further, ultracapacitors act alongside batteries or standalone storage to take care of transients on grid supply systems. Inverters use them in place of batteries for fast and reliable response in case of power failure to allow industries an orderly shutdown or a switchover to another supply source, which is particularly important in continuous process industries.

Buses are running on ultracapacitors in China since 2014, and now in some European countries and the US. These are periodically charged en route at stops under 30 seconds. Even trams powered solely by ultracapacitors are now running in China. Trams in Europe use ultracapacitors for short distances or for tiding over gaps without overhead catenaries. Ultracapacitors have been used by themselves or in hybrid vehicles for public transport, either along with IC engines or with batteries in many countries.

New developments in ultracapacitors are driving their energy density and/ or power density upwards with the additional benefits of minimum maintenance and durability. Ultracapacitors have made substantial inroads into motor-racing. The Toyota TS030 Hybrid uses a hybrid drive train with ultracapacitors (no battery) and IC engine and won three of the 8 races in the 2012 FIA World Endurance Championship season. Gasoline based hybrid cars are using ultracapacitors to get improved performance and efficiency.

Electric and hybrid electric vehicles using ultracapacitor/battery combinations are well known by now, and a reduction in fuel consumption ranging 20 to 60 % has been claimed by recovering brake energy. The short charging time, stable electrical properties, broad temperature range and longer lifetimes are obvious advantages. There is good scope for improvement in increasing energy and power density, and reducing weight, volume and cost.

H] Flywheel Energy Storage

Flywheel energy storage systems (FESS) employ kinetic energy stored in a rotating mass with very low frictional losses. Electric energy input accelerates a flywheel to very high speeds of tens of thousands of RPM via an integrated motor-generator. Energy is discharged by drawing kinetic energy using the same motor-generator. These modern flywheels are way different from conventional flywheels made of iron mass, rotating at not over 8000 rpm.

Advanced FESS offer unmatched energy density, high efficiency with miniscule losses, and are best suited for periods from few minutes to several hours. These high speed flywheels specially developed for high energy storage (FESS) rotate at speeds of 10,000 to 100,000 RPM. Special materials are needed to withstand tremendous centrifugal forces and stresses at these speeds, and tensile strength is more important than density of material. Main features include a high speed flywheel made of fiber glass resins or polymer materials with a high strength-to-weight ratio, operating in vacuum to eliminate aerodynamic drag. The flywheel spins is supported on frictionless magnetic bearings, and the whole assembly is enclosed inside a high vacuum chamber. Peripheral speeds are over 1000 m/s, and usual engineering materials are not capable of sustaining the resultant stresses.

Figure 6: Comparison of Flywheel Vs. Battery (Source: Electropaedia)

Excess electric energy from supply via motor-generator accelerates flywheel to high speed and energy gets stored in it. When energy is drawn, the motor-generator reverses in direction. Flywheels are used in inverters and grid systems for large energy storage, with very fast charge / discharge times, not possible by any other storage source (except ultracapacitors). Energy density is as high as 100-130WH/Kg four times or more compared to lead acid battery. Even the volume energy density is much higher than lead acid battery. Rapid charging of a system occurs in less than 15 minutes. Round-trip efficiency can be as high as 90% or more, and capacities range from 3 KWH to 130KWH. Battery of flywheels can be used to scale up storage capacity.

Flywheels from Beacon Power spin up to 16 000 rpm, with a maximum storage capacity of 25 kWh that can be delivered back to the grid at maximum power rate of 100 kW (over 15 minutes). Number of these flywheels combine in a system to get desired storage capacity. Such flywheel farm is deployed primarily as an electricity frequency stabilizer, and to store cheap electricity available in the grid during the night.

Advanced flywheels are used for protecting against interruptions to national electricity grid. They provide power during period between any loss of utility power and its return, or the start of a back-up power system (e.g. a DG set). Flywheels can discharge at 100 kW for 15 seconds and recharge immediately at similar speeds, providing 1-30 seconds of ride-through time. Back-up generators are typically online within 5-20 seconds.

A flywheel storage plant for grid power storage with a capacity of 5MWh, providing a power output of 20 MW for over 15 minutes has been installed at a Beacon Power plant in New York and other large installations are in the pipeline. Such installations on smaller scale can take care of industry needs. Already several manufacturers around the world are supplying flywheel systems.

Volvo envisions hybrid vehicles with FESS systems to produce huge power for a few seconds to get a car to speed up, or accelerate fast when needed. They replace the battery pack and electric motor in hybrid vehicles. An experimental system on a turbocharged four-cylinder Volvo S60 accelerates to 100 km/h in 5.5 seconds. Kinetic energy recovery system (KERS) adds up to 80 HP, with 25% better fuel economy than conventional system with two more cylinders. Volvo flywheel is a carbon fiber disc about 20 cm in diameter, weighing 6 kg, housed in vacuum.

Conclusion

It is seen that a number of energy storage technologies – both existing and new developments will be available and at hand in sufficient measure worldwide in very near future. These will serve changing needs of electronic and auto industry, both short term and long term altering energy storage needs in supply grids, as well as renewable energies. With developments going on in renewable energies and energy storage (and both becoming economical), the world should be substantially free of dependence on fossil fuels in near future.

If you want to share thoughts or feedback then please leave a comment below.